In a steel mill, every piece of equipment is designed to have a certain flow-through capacity or size limitation. Some of the equipment size limits are dependent upon the external dimensions of the material passing therethrough; others are dependent upon weight or mass of material passing therethrough.
In a rolling mill comprising a series of sequential rolling stands designed to accept the initial slab or cast strand and to process it without interruption until final end-product thickness is reached, it is essential that each downstream item of equipment have an operating capacity that is sufficient to handle the incoming material from upstream. This requirement admits of very little flexibility in the mill operation for any given end-product. On the other hand, if a Steckel mill is used to perform all of the rolling (or perhaps all of the rolling following an initial roughing mill reduction or the like) then selection of different slab weights and lengths is possible. The rolling schedule for a Steckel mill may be subject to considerable variation, and it is not always the same piece of equipment that determines the operating capacity or limit for the rolling mill, as a whole, especially if the mill is designed to produce both plate and strip.
Consider, for example, an in-line Steckel mill configuration in which the Steckel mill is to roll a sequence of slabs fed from a reheat walking beam furnace that is capable of handling slabs of 6" thickness, 120" wide and up to 75' long. Such slabs of maximum dimensions weigh approximately 92 tons. The Steckel mill operates in conjunction with a pair of associated coiler furnaces, each of which has an upper limit on its capacity, usually determined by weight. Suppose that each coiler furnace can handle coiled strip or plate up to 75 tons weight. If strip is the end-product, then the strip has to be coiled in a downcoiler or upcoiler, which itself will have a weight limit for a full coil of strip. Suppose that the weight limit of the downcoiler is 371/2 tons.
It can be seen that the result of the foregoing limitations is that if the mill operator is asked to produce plate product, he cannot cope downstream with a single slab of 92 tons and hope to use the coiler furnaces for such slab. So he will, in accordance with conventional practice, elect to cut not a 92-ton slab but rather a 75-ton slab, which will be within the limits of capacity of the coiler furnaces used with the Steckel mill. On the other hand, if the mill operator is required to produce strip product, the cut slab is typically limited to 371/2 tons, because that is the upper limit of the downcoiler (or upcoiler) capacity. The result is that the mill is not used in an optimally efficient manner, in that the walking beam furnace and Steckel mill, apart from weight limitations of the coiler furnaces and strip coiler devices (and possibly other mill limitations, such as available length of rolling run-out tables) could work with a 92-ton slab, but, in the case where strip is produced, the mill operator is, in accordance with conventional practice, limited to slabs of 371/2 tons, and where plate is produced and the intermediate product is coiled in the coiler furnaces, the operator's slab size limit is 75 tons.